No matter the make and model, your car is essentially a rather heavy mass of metal, plastic, rubber, glass and fabric. When all of that weight is moving at speed, physics dictates that a lot of different forces will be acting upon you and your car. Your suspension’s job is to counter some of those forces, and redirect or absorb others. If for example, you are turning a corner and the road is a little bumpy, the car might tend to lean over and bounce somewhat, meaning that your tyres will lose grip on the road surface. As such, your suspension will have been engineered to accommodate the countless combinations of surfaces and movements that go into even the most mild everyday driving, and ensure that rubber meets road no matter what. Of course, like everything, suspension isn’t without its limits - nor is it without the potential for things to go wrong, but with a little knowledge of what is going on underneath your car, and with a bit of awareness as to the type of setup you have, and the condition it’s in, you can minimise the risk and enjoy a safe and comfortable driving experience - whether you are commuting to work on city streets, blasting along cross country tracks in the outback, or making your own tracks whilst rock-crawling in the ranges.
How does suspension work?
Suspension is a catch-all term for all of the bits that move around (or stay put) to ensure that your wheels contact with the road surface and your car doesn’t float about uncontrollably. It is responsible for cushioning your ride, balancing the weight of your car, and for allowing the correct angles, heights and movements of your wheels when you are moving, stopping and turning. Generally speaking, all suspension setups consist of a series of springs, dampers and bars all tied to one another and your car’s body, and whilst the differences between the various designs can be quite a long list, the basic principles remain the same. The main focus that most manufacturers have when designing suspension can be broken down into two main areas - Ride and handling.
Ride is concerned with how the car feels when you are traveling over uneven surfaces. It is a focus on driver and passenger comfort. Handling is more to do with safety, being concerned with the car’s ability to corner, stop and drive in a safe and controllable manner. As mentioned above, the three main components in any modern suspension setup are springs, bars and dampers. All three have a part to play in maintaining good handling and a comfortable ride.
Springs usually come in one of four main forms, with each being found most commonly on different types of vehicle. The job of any suspension spring though is the same - to absorb the vertical movement of the wheels, and to suspend the body of the car above the road surface.
By far the most common type of spring found in modern cars is the Coil Spring. They are the type of spring that people generally think of when they hear the word outside of the weather channel, and as the name suggests, they are simply a length of metal that has been coiled around an axis. They are the same design that is found in trampolines, ballpoint pens and pogo-sticks, however in automotive use, each spring is made from heavy-duty metal, forged specifically to bend back to its original shape. This material is called “torsion bar” and also happens to be the same term as is used for another type of spring setup.
Torsion Bar setups are constructed from a bar of the same kind of steel as is used in coil springs, but that hasn’t been wound up. Rather the bar is attached at one end to the vehicle’s body, and to a metal wishbone at the other. The wishbone is attached to the hub that holds the wheel and acts as a lever when the wheel is moved up and down. This levering action then twists the torsion bar, which due to its specific composition, provides a springing force. This type of setup is commonly found in older, European cars and whilst it was the mainstay for some time, it has generally been superseded by coil springs.
Probably one of the most widely used setups in terms of the outright length of time that it has been around, is the Leaf Spring. Leaf springs are a series of bars, anchored at either end and bound together in such a way as to flex as one unit - they use the metal’s ability to spring back to shape in a bending motion, as opposed to the twisting of coil and torsion bar setups. Leaf springs are one of, if not the oldest form of suspension - having been used on horse-drawn carriages, but are still in widespread use today on heavy duty vehicles, or where cheap but rugged suspension is required. Most utes and trucks have leaf sprung rear suspension, even if more advanced setups are used up front.
The last type of spring that is used (if not as commonly as the others) is the Air Spring. Like leaf springs, the design is actually fairly old - having been used on high-end horse-drawn carts, and following on from this luxury use, the spring type is most commonly found in older luxury cars, or as an additional spring type in newer cars. It works by utilising the compression of air inside little cylinders (usually rubber) which will normally be inflated or deflated through the use of an air compressor in order to control both stiffness, and ride-height.
The springs in any particular car are designed to balance the two principles of suspension (ride & handling) with the stiffer the spring, the more direct and responsive handling characteristics offered. This will generally reduce the comfort of the ride however, since stiffer springs will reduce the amount of energy that can be absorbed. Usually sportier cars will forego some of the comfort and smoothness of their ride in order to allow for better handling, and luxury cars will tend towards the opposite end of the spectrum. This balancing act is one that is constantly refined and revised by engineers, who will aim to offer the best of both worlds. No matter how the springs are designed though, it is all for nothing if there isn’t something in place to dissipate the energy that the springs absorb.
This is where dampers come in.
It isn’t likely that you’ll have heard of them by that name, but rather you probably know of them as shock absorbers or simply, shocks. If your car didn’t have shock absorbers to dampen the forces that act upon the springs, then the springs would bounce up and down until they reached a natural stopping point and as a result, so would your car. Instead, shock absorbers work to absorb the energy generated by the vertical movements of your car’s wheels as they move up and down over the road surface, or the force applied to the springs by the car’s body when accelerating or stopping, therefore letting the springs take the initial energy, but preventing them from bouncing uncontrollably. Most modern shocks only come in one basic form, called a twin-tube design. It is as the name suggests, constructed from two tubes - one inside the other. One end of the shock absorber is attached to the frame of the car and the other is attached to the axle or arm that carries the wheel. When the springs coil and uncoil the energy is transferred down through the top mount, along a piston shaft and to a piston. This piston moves down through the inner tube (called the pressure tube) which is filled with hydraulic fluid, or shock oil. The piston itself has small holes in it, that are designed to let a specific amount of fluid through and the resistance to the piston moving is what absorbs the energy that was transferred through the whole assembly. Since only a specific amount of fluid is let through the holes in the piston, the remaining fluid is pressurised - which increases its ability to absorb the energy and therefore provides the ability to respond differently based on the velocity of the piston travelling down the pressure tube. This in turn means that your shocks will behave differently under different loads and will be able to absorb the energy of sudden jolts, as well as more gentle force such as body roll from cornering, braking and acceleration. Throughout all of this, the job of the reserve tube is to store the excess hydraulic fluid, and to act as a buffer to prevent the fluid from blowing through the seals around the piston under extreme loads.
A slightly more advanced version of shock absorbers are struts, or more properly “Macpherson struts” after their original designer. They are essentially a shock absorber mounted within a coil spring, and do the job of both components at the same time. They are one of the most common spring and shock combinations found on modern vehicles, and there are plenty of aftermarket options that are built using this design. Some of the most widespread aftermarket substitutes for a strut setup are referred to as “coil-overs” which is short for “coil over shock” and have some slight differences from the standard Macpherson strut setup, but for all intents and purposes function in much the same way..
Bars and Bushings
As mentioned above, your suspension consists of some sort of spring setup as well as something to absorb the forces that act upon the springs. This is all well and good, but there is a third major component that is needed to tie everything together, and ensure that each corner works in concert. This is the job of the bar setup, which is known by a few names - anti-roll bars, and anti-sway bars or simply roll-bars and sway-bars. The bar’s job is to transfer the motion from one side of your car to the other, and ensure that when the shocks and springs on one side move up and down, that the other side responds accordingly. This is done to make certain that the wheels on both sides maintain adequate contact with the road surface, and to minimise the “sway” or “body-roll” of your car when cornering. The bars themselves are usually made of a certain type of metal, and in a specific design, to allow a particular degree of flex, but not so much as to be pointless. There are a wide variety of setups available, and each will differ depending on the intended purpose of the car. Most passenger cars however, have fairly simple setups front and rear.
In addition to the sway-bars, or roll-bars, there are various other arms, bars and linkages that all work to ensure that each component of your driveline and suspension sits in the right place, and each of these, as well as the shocks and (depending on the setup) the springs, are mounted in such a way as to minimise the noise and wear from their operation.
The bits that do this job are called bushings (or more commonly bushes, though that’s not actually the correct term.) They are usually made from rubber or polyurethane, and sit in the joints between the various suspension parts in order to absorb vibrations and friction, but still allow for movement. Almost all cars will have some sort of isolator bushing to prevent metal-on-metal contact, and it is important that these be maintained since wear or damage can not only result in noisy operation of your suspension, but also unintended extra flex and movement of the various components.
As you may gather, there’s a little more to your suspension than many people assume, and so it is a good idea to check as to which type your car has and the condition of the various parts. If there is too much movement (or play) in the suspension components due to worn bushings, then your car will likely creak, groan or make knocking sounds. If you have worn and sagging springs, then your car may wallow around corners, or scrape on bumps, and if you have tired or leaky shocks, then your car might bounce uncontrollably, or move erratically when you drive over rough roads. Anything that goes wrong with your suspension, or any wear that isn’t addressed as soon as possible, will cause your car to ride badly and handle badly - negating all the hard work of the engineers that designed the whole thing!
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